In November 2012 and March 2014, I posted a blog and White Paper, respectively, on iron deficiency chlorosis (IDC) that affects soybeans grown on the high-pH soils in East Mississippi. At that time, there was little or no research that addressed the determination of a solution to this problem, and the above articles concluded that the best strategy for managing IDC is to select a soybean variety with tolerance.

In Vol. 106, Issue 4 of Agronomy Journal, results from a study entitled “Iron Chelates Alleviate Iron Chlorosis in Soybean on High pH Soils” that was conducted in the Blackbelt region of Alabama are presented by authors A. V. Gamble et al. These findings shed new light on how this problem can be managed in affected fields.

Study Factors

The primary purposes of the study were to: 1) determine the effectiveness of Iron (Fe) chelates and supplements for alleviating IDC in soybeans grown on high-pH soils; and 2) compare visual ratings of IDC with remote sensing and chlorophyll meters for determining level of chlorosis in affected soybean plants.

Two sites in the Blackbelt region of Alabama were used–one on a Sumter soil series with an average pH of 8.2, and one on a Leeper soil series with an average pH of 7.9.

The study was conducted from 2010-2012.

Planting dates were in mid-May.

The same moderately IDC-sensitive MG 5.5 variety was used for the duration of the study.

Eight iron treatments and an untreated control were used.

• Fe-EDDHA (6% iron) applied at 2, 3, and 4 lb/acre in-furrow at planting


• Fe-EDDHA applied at 2 lb/acre as a foliar spray at V3


• Fe-EDDHA applied at 4 lb/acre as a split application divided equally between in-furrow at planting and as a foliar spray at V3


• Fe-Citrate (20% iron) applied at 2 lb/acre in-furrow at planting


• Fe-Citrate applied at 2 lb/acre as a split application divided equally between in-furrow at planting and as a foliar spray at V3


• FeSO₄ (20% iron) applied at 4 lb/acre as a foliar spray at V3

Chlorosis was evaluated at the V3 and V5 stages using: 1) Visual Chlorosis Score (VCS) rating made by the same person for the duration of the study, where 1 = no chlorosis and 10 = severe chlorosis with stunted growth and necrosis or death of plant; 2) a chlorophyll meter; and 3) the Normal Difference Vegetation Index (NDVI) by measuring reflectance above the crop canopy.

Results

VCS ranged from 3.8 to 6.6 at the Sumter soil site and 2.8 to 4.6 at the Leeper soil site. In-furrow treatments with Fe-EDDHA at planting resulted in the lowest VCS scores, but VCS ratings were not lowered enough to reduce chlorosis level to that of a non-chlorotic plant. A rate higher than that used in this study may be required to do this, but higher product rates will affect net returns and may not be economically feasible.

Improvements in VCS were more common at the V5 growth stage sampling.

VCS ratings were more effective for assessing chlorosis than were NDVI and chlorophyll meter measurements. NDVI measurements were likely affected by the minimal ground cover provided by the plants at the early sampling dates used in this study.

Fe-EDDHA applied at 4 lb/acre either in-furrow or as a split application was effective in improving yield. Average yield increase for the best treatment was 3.25 bu/acre above the average 16.7 bu/acre for the untreated control.

Soybean prices used in this study were $11.17, $11.99, and $14.71 per bushel in 2010, 2011, and 2012, respectively. Fe-EDDHA price was $6.82/lb, or $27.28 for the 4 lb/acre rate. Thus, returns were increased by about $9 to $20.50/acre across the 3 years using the 3.25 bu/acre best yield increase measured in this study.

Using the yield increase of 3.25 bu/acre and the Fe-EDDHA cost of $27.28/acre for the 4 lb/acre rate used in this study, soybean price will have to be above about $8.40/bu for this to be a profitable treatment to alleviate IDC in soybeans.

Assessment of Results

The significant finding from this study is that Fe-EDDHA applied in-furrow at planting or as a split application between in-furrow at planting and a foliar spray at V3 can improve yield when IDC-sensitive soybeans are grown on soils that promote IDC.

The magnitude of the yield effect measured in this study should be determined in a higher yielding environment than used in this study, where yields were in the 16.5 to 20 bu/acre range. In other words, will the yield effect be greater as yields increase, or will it remain the same regardless of the yield level?

The findings from this study should be confirmed on several varieties that are known to be IDC-sensitive, and/or that are known to have varying degrees of IDC sensitivity among them. This can be done on a known IDC site with varieties that have a confirmed history of IDC sensitivity.

Composed by Larry G. Heatherly, Aug. 2014, larryheatherly@bellsouth.net